In the recovery of hydrocarbons, such as oil and gas, from natural hydrocarbon reservoirs, extensive use is made of wellbore fluids such as drilling fluids, completion fluids, work over fluids, packer fluids, fracturing fluids, conformance or permeability control fluids and the like.
In many cases significant components of wellbore fluids are thickening agents, usually based on polymers or viscoelastic surfactants, which serve to control the viscosity of the fluids. Typical viscoelastic surfactants are N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride and potassium oleate, solutions of which form gels when mixed with corresponding activators such as sodium salicylate and potassium chloride.
The surfactant molecules are characterized by having one long hydrocarbon chain per surfactant headgroup. In the viscoelastic gelled state these molecules aggregate into worm-like micelles. Gel breakdown occurs rapidly when the fluid contacts hydrocarbons which cause the micelles to change structure or disband.
In practical terms the surfactants act as reversible thickening agents so that, on placement in subterranean reservoir formations, the viscosity of a wellbore fluid containing such a surfactant varies significantly between water- or hydrocarbon-bearing zones of the formations. In this way the fluid is able preferentially to penetrate hydrocarbon-bearing zones.
The use of viscoelastic surfactants for fracturing subterranean formations is discussed in EP-A-0835983.
A problem associated with the use of viscoelastic surfactants is that stable oil-in-water emulsions are often formed between the low viscosity surfactant solution (i.e. broken gel) and the reservoir hydrocarbons. As a consequence, a clean separation of the two phases can be difficult to achieve, complicating clean up of wellbore fluids. Such emulsions are believed to form because conventional wellbore fluid viscoelastic surfactants have little or no solubility in organic solvents.
A few anionic surfactants exhibit high solubility in hydrocarbons but low solubility in aqueous solutions. A well known example is sodium bis(2-ethylhexyl) sulphosuccinate, commonly termed aerosol OT or AOT (see K. M. Manoj et al., Langmuir, 12, 4068-4072, (1996)). However, AOT does not form viscoelastic solutions in aqueous media, e.g. the addition of salt causes precipitation.
A number of cationic surfactants, based on quaternary ammonium and phosphonium salts, are known to exhibit solubility in water and hydrocarbons and as such are frequently used as phase-transfer catalysts (see C. M. Starks et al., Phase-Transfer Catalysis, pp. 125-153, Chapman and Hall, New York (1994)). However, those cationic surfactants which form viscoelastic solutions in aqueous media are poorly soluble in hydrocarbons, and are characterized by values of Kow very close to zero, Kow being the partition coefficient for a surfactant in oil and water (Kow=CoCw, where Co and Cw are respectively the surfactant concentrations in oil and water). Kow may be determined by various analytical techniques, see e.g. M. A. Sharaf, D. L. Illman and B. R. Kowalski, Chemometrics, Wiley Interscience, (1986), ISBN 0471-83106-9.
Typically, high solubility of the cationic surfactant in hydrocarbon solvents is promoted by multiple long-chain alkyl groups attached to the head group, as found e.g. in hexadecyltributylphosphonium and trioctylmethylammonium ions. In contrast, cationic surfactants which form viscoelastic solutions generally have only one long unbranched hydrocarbon chain per surfactant headgroup.
The conflict between the structural requirements for achieving solubility in hydrocarbons and for the formation of viscoelastic solutions generally results in only one of these properties being achieved.